Shielded CAT 5 perfect, signal in close proximity to its return path, impedance similar to the signal on the PCB. As Qusp stated. The impedance should match from the transmitter to the reciever. Otherwise you will degrade the signal and add jitter to the clock.
Silver, what is that going to do to the digital signal transmission.
Of course the only way you can revue the cables effect on the signals will be with scope shots and eye daigrams
DIGITAL is DIGITAL, read the two excellent books by Howard Johnson, they explain in great detail on how we can propogate digital signals down traces, along wires etc and still get the correct bit pattern.

Shielded CAT 5 perfect, signal in close proximity to its return path, impedance similar to the signal on the PCB. As Qusp stated. The impedance should match from the transmitter to the reciever. Otherwise you will degrade the signal and add jitter to the clock.
Silver, what is that going to do to the digital signal transmission.
Of course the only way you can revue the cables effect on the signals will be with scope shots and eye daigrams
DIGITAL is DIGITAL, read the two excellent books by Howard Johnson, they explain in great detail on how we can propogate digital signals down traces, along wires etc and still get the correct bit pattern.

I am sorry but digital is not digital and the idea that you will get bit perfect copies is just is not convincing. Cat 5 cable sounds different to Cat 6 . To my ears in my system Cat 6 even if just replaced by 1 metre lengths sounds better. You can tell me and show me all the proofs that you want that this is impossible but the difference was not subtle and I am not going back to Cat 5.

With regard to this cable my idea is very simple Silver cable is a much more efficient and better conductor than copper the difference in cost is negligable so I went for the silver. I was going to consider getting the cable fitted with shield which is an option (£5 extra) but then companies are using Cat 6 cable to connect I2s and that has no shield so I decided to take a chance. Sadly not got the cable back yet so no update on the sound but I do promise that I will post what I think .

With digital, the slightly less resistance of silver vs copper is not realy taht relevant, what is is controlling the impedance of the path the signal travels down, and matching the driver to the line, with either series or parallel termination. In digital signal transmission the main area of concern is the brief interval of time when the signal changes, thats when all the excitement happens. This is the rise time of the signal and the length (electrical) of the trace to the reciever(s). get that part of the equation right and the rest will follow.
If digital is not digital, what is it? The whole point of the ever increasing world of digital data transmission is its reliability, it not affected by noise like analogue transmission, never mind the other benefits. At the end of the day the reciever will either register a 0 or a 1, no inbetweens, unlike analogue that may register a slight change in voltage due to PCB or Cable effects.
Everything else digital we use every day seems to work quite well...why is audio repreduction so flawed in the digital domain, not saying it is perfect, but is it as bad as made out.
I use a couple of squeezeboxes unmodified through a mismatch of amps and speakers, in different rooms, but it all sounds good to me, better than wearing out CD's trying to get them in the player, is it better sound quality ( I have a dedicated music server, basic, big disk, runs SB server) or is it the ease of finding a CD and enjoying the music!

Please excuse me as my technical knowledge is not anyhting like as good as it should be but the digital is either 0 or 1 is not correct either I have been reading a huge number of articles on this at various sites dealing with computer audio and others and the problem is that while the information is only 1 or 0 the bit word is quite long combination of these words and the digital system is designed to accept certain words with minor (?) errors and then recalculate them to make up what it thinks was sent orginally.

I think we have to accept that digital is not perfect and is subject to errors and these errors are small enough to be accepted and not have the item just cut out but they are not the same. If all digital was as it should be then various Dac,s Streamers and other items would not all sound different. I know there are people who say they do not sound different and I am sure they state this becasue they do not hear any difference I wish I was like them then I could just buy the best I can afford and play music . But I can hear differences and soem of them are not subtle . To date I have heard almost the whole range of Nait equipment and find them all not to my tastes I have no idea why they are well designed well made and lots of people are fervent in their admiration. Yet they leave me with an empty feeling that just wants to turn the music off for they do not image and they are just too relentless .
I wish I had the real technical knowledge to understand and argue my case from a real engineering point of view but I can not . I also think there is much in this field that is not fully understood as well by those who do have the technical skill so I would suggest that we accept that we will have differing views on this because of my inability and ignorace in being able to explain what I hear.

Digital is 1s and 0s, look up how digital works, sorry but I have spent nearly 3 decades laying PCBs out and working in design teams for a whole range of projects from the mundane to highly technical. i also now do a lot of high speed routing and training for PCB designers in high speed digital layout and using the tools for such, and its all geared to transporting bits and getting from a to b intact and at the right time.
The early days od CD's and digital in music was quite problematic, hence Reed Solomon correction code, and the Audio belief in digital being flawed probably comes from this. And a lot of the early problems were the reading of the disk. These days and for many years digital (while not foolproof) has enough checking systems to ensure that the digital data transmission is perfect from a to b, otherwise so many things would not work, the internet for one.
i am not disputing any sound differences that may occur with different equipement, but digital data transmission is well understood by those working in the field, and even mundane products have to work and work reliably these days, never mind life critical applications, where failure is not an option.

@marce - you don't get the concept of whishfull thinking?
One's brain can imagine things just based on what wishes/desires are firing the synapsis... Like the conviction that silver would sound better than copper for digital transmission lines - it would sound "better" to somebody that really, really wants to be that way

I know thats why I work in an engineering disipline...and probably why it took 2+ years and many cabinets (and different drivers)before I was finaly happy with some OB speakers (and why I dont MOD digital).
Even if I thought I was percieving a difference I would have to prove it to my self with measurements and then it would have to be repeatable
With I2S (and I2C) personaly I would be wary of taking it off board, having done numerous boards with Ti codecs (TPS320, TVS320 etc) and other devices, layout and impedance control can be critical, and matching the drivers output to the layout and number of devices on the local bus is critical.
I dont think moders who are not involved with the day to day design of digital (and analogue) circuitry realise the intense level of development that goes into such designs. They want to spend a week simulating and laying out a JTAG clocks to a multi device board to see how **** retentitive you have to be with digital, or building models of scope probes for the simulation software, so you can match the simulations to the viewed waveform (often distorted by the loading of the probe).
The only time I use silver plated copper wire (instead of tinned) is for certain applications that require a better temp rating etc, but these aint commercial designs.